Saturday, November 5, 2016

"How Water Can Identify Murder Victims and Fake Scotch"

From Nautil.us:

These maps pinpoint an object’s origins by the isotope ratio in the water.

One of the criteria that make a Scotch a Scotch is that it’s made with
Scottish water. That means that a clever connoisseur should be able to
tell whether her drink is authentic by tracing the source of its H2O
molecules. But how? After it’s collected and filtered of impurities,
water is water, right?

Not exactly. The elements hydrogen and
oxygen appear in nature in various forms, called isotopes, which differ
in neutron number. By measuring the relative abundance of these isotopes
in a water sample using a technique called mass spectrometry,
scientists can determine its “isotope ratio.” And as it turns out, the
isotope ratio of water varies dramatically from place to place.

Scientists
first noticed this in the 1950s, when they started collecting
freshwater samples from around the world, says Gabe Bowen, a geochemist
at the University of Utah. What’s more, he says, researchers realized
isotope ratios “varied in predictable ways.” The proportion of heavier
water isotopes decreased at higher latitudes, higher elevations, and
further from the coasts.

Water weights:
The isotopic ratio of hydrogen in rain and snow varies across North
America, with higher ratios

of heavy isotopes (reds and oranges) at
lower latitudes, lower elevations, and close to the coasts; and lower

ratios (greens and blues) at higher latitudes, higher elevations, and
deeper into the interior of the continent.

By the 1990s, the U.S. Geological Survey had amassed a collection of
nearly 5,000 isotope measurements of river water and precipitation from
around the country. When hydrologist Carol Kendall and her colleague
Tyler Coplen analyzed these data, they confirmed the large-scale pattern
of water isotopes across the U.S. Then, they used their measurements to
create a map of the isotopes of water at every location in the country.
The result, published in 2001, was a detailed national isoscape—a topographic map that showed isotope ratios instead of elevation.

Since
then, researchers like Bowen have created computer-generated isoscapes
for different elements and diverse geographic regions, from a single
watershed to the entire planet, using knowledge about atmospheric
circulation, rainfall patterns, and other physical processes to make the
best estimates for unmeasured areas. Hydrologists and forensic
scientists can then use these maps to trace water molecules back to
their origins—even after they’ve been absorbed by the cells of plants
and animals, for example, or turned into distilled alcohol.

One promising application of isoscapes involves the study of water
cycles and movement. By comparing isotopes in groundwater samples with
precipitation isoscapes, researchers can determine where and when the
world’s aquifers get recharged. And in a 2014 study, Stephen Good, at
Oregon State University, and his colleagues used isotopic differences
between tap water and local precipitation to determine where people in
the American West get their water. (You might think we would already
know this information, but in fact, the federal government hasn’t
tallied up Western water transfers since the 1980s, and there’s no
comprehensive database of municipal water sources.)